－>　Main Products--ZGM Roller Grinding mills ：

1. General Outline

BEIJING POWER EQUIPMENT GROUP (BPEG) is the leading manufacturer of mills in China. BPEG has been manufacturing mills since the end of 1950's. They are therefore one of our oldest products.

Today an extensive range of them is available for a great variety of industrial engineering and these mills are, in most instances, designed to the customer's requirements. BPEG's product spectrum includes, for instance, medium speed mills and low speed tube ball mills and so on. For new plants and techniques, BPEG has frequently been a pioneer (trend-setter) in the planning and developing of the required powder making engineering.

2. Technical Milestones in Development and Design

Creativity and the technical innovations that went with it have always been very highly rated at BPEG. Substantial progress in powder making technology has been achieved in over 40 years. Some of the outstanding milestones along the way were:

1950's Construction of rolling ball type mill (E type mill)
1960's Production of plate type mill
1970's Manufacture of rolling ball type mill (ZQM series)
1980 Start to manufacture of roller type mill (ZGM series)
1985 MPS mill license agreement with DEUTSCHE BABCOCK
1991 ZGM113 (MPS225), Output of mill 87.7t/h
1992 ZGM123, Output of mill 107.6t/h
1998 ZGM133, Output of mill 129.9t/h

Integrated CAD system and complex computer programs are being used for product design. They accurately meet our customers' requirement. New products are being developed in close co-operation with technical colleges and institutes, such as, Tsinghua University, Huazhong University of Science and Technology, as well as Thermal Power Research Institute of China.

3. State-of-the-Art in Manufacture and Quality

High product quality is assured by State-of-the-Art and well-tended facilities. In our workshops, the most advanced machine tools and testing facilities are equipped with that ensure the acknowledged quality of our products.

Examples are:

-Case hardening of bevel gear and spur gear.
-Gear machining with KLINGELNBERG Cutting Machine and MAAG
Grinding Machine.
-Gear measuring with 3-coordinate measuring machine.
-Castings produced by production line with resin sand.
-Blanking line by CNC control.

Quality tests involve consistent monitoring throughout the manufacture of all components, i.e., from raw materials and sub-assemblies to the final product. This system ensures the availability and long life of our products.

Our quality assurance system has been certified for many years. The quality standard has been updated by ISO9001-2000 system, which, in its overall approach, covers all functional areas of our company. The respective certificate is only regranted after the system has been vetted again by a neutral institution. We have received this certificate.

4. ZGM roller vertical mill

4.1 Application

For all grades-ranging from hard lignite to anthracite. ZGM vertical mills have a firmly established reputation for economy in the preparatory processing of coal. They are used mainly in direct pulverized coal firing systems or steam boilers in power plant. They are also used for supplying pulverized fuel to industrial furnaces in steel works, to rotary cement kilns in cement works and coal gasification plants.

4.2 Grinding Principle

A grinding table rotating round a vertical axis is driven by a planetary gearbox. The grinding rollers, mounted in fixed positions, are pressed down on the table under loading. The axes of the grinding rollers are inclined at 12°~15°, in combination with the grinding surface of the table they achieve optimum grinding effect, as experience in actual plant operation shows.

The big grinding roller performs not only a purely rolling motion, but also a sliding motion because the roller axis and grinding table axis do not intersect in the plane of the table. The differential speeds due to this configuration are a desirable feature, producing an additional comminuting action by the shearing forces developed.

Large-diameter rollers can be arranged to run on a small surface area of the grinding table. Bearing in mind that the mill throughput rate increases in proportion to the square of the roller diameter. The advantage of using big rollers will be evident.

4.3 Operation and design features

The raw coal is fed centrally from above through the classifier mounted over the mill or from one side onto the rotating grinding table. On passing under the rollers the coal is ground by their action assisted by the pressure of the hydropneumatic spring loading of the roller. When the roller travels over the bed of material on the table, a system of loading frame and rods connected to the working piston of a hydraulic cylinder causes oil to be displaced from the space below the piston into hydropneunatic accumulators.

The pulverized coal is flung by centrifugal force into the zone over the nozzle ring (louver air ring) surrounding the table. Where the coal particles are swept upwards by the stream of hot air into the classifier. The intimate contact with the hot air causes the moisture contained in the coal to evaporate spontaneously, so that the desired exit temperature of the pulverized product from the mill (70°~120°) is already attained in the grinding chamber. The oversize particles rejected by the classifier fall back onto the grinding table for further size reduction. The fine particles, i.e., the finished product, are carried out of the classifier with the air.

The mill is driven by a special motor through a planetary gearbox. A segmental thrust bearing in the gearbox absorbs the forces exerted by the rollers. Before the mill drive is started, the grinding rollers are lifted hydraulically off the grinding table. This action is performed by oil pressure diverted from the cylinders exerting the spring loading. In this way, the mill can be started, either empty or filled with coal, with only a small starting torque, corresponding to about 40 percent of the rated torque. The special motor that can be started direct-on-line is adequate. No auxiliary drive is necessary.

4.4 Range of types

ZGM mills equipped with three rollers are used for coal grinding. The increase of radius of grinding table is applied in mills with a mean radius of grinding table from 650mm upwards.

4.5 Noise, Vibrations, Fire prevention

Industrial health and safety regulations lay down requirements with regard to noise emission, mechanical vibrations and safety of coal grinding mills.

In the power generating industry, the sound pressure level of a mill system, measured at a distance of one meter from it, is not allowed to exceed 85dB(A) under full load or part load operation or under starting or stopping conditions. In addition, these are regulations for limiting the vibration velocity of mill and building foundations over the whole load range, as well as safety regulations for the prevention of fires and explosions in coal grinding systems.

To meet these requirements, ZGM roller grinding mill incorporate various constructional design and control engineering features:

-Extremely quiet running is obtained because each grinding roller is individually mounted and restrained in the loading frame in combination with gentle hydropneumatic spring loading.
-Vibrations are prevented by raising and lowering the rollers, timed in co-ordination with the raw coal feed, on starting and stopping the mill.
-Movable clearing devices installed under the grinding table keep the gas flow duct free from foreign bodies and material that settles there.
-As all the internal surfaces of the grinding chamber are vertical or sloping, no deposits of coal can form on them.

4.6 Safety and plant layouts

Stored raw coal and pulverized coal may undergo self-ignition. The explosion hazard associated with coal dust and pulverized coal has been dealt with in many publications.

Active safety measures to protect coal-grinding plants can be applied. Active measures against explosion hazard comprise operating the system under inertized internal atmosphere (with or without a separately introduced inert gas), avoidance of sources of ignition, and using simple and clear plant layouts.

4.7 Rotary classifier

The rotary classifier is equipped with a bladed rotor. Product fineness is governed mainly by the rotor speed, which can be varied to suit requirements. This type of classifier is used in cases where fineness corresponding to less than 10 percent retained on the 0.09mm sieve is required or the product must have a narrow grain size distribution.

4.8 Coal feeding

The feeder for supplying the coal to the mill must ensure a satisfactory, uniform and controllable flow of material.

There are dry coals with good flow properties, which can be extracted from the bins by means of rotary gate or table feeder. For coals with poor flow ability, however, continuous flow feeders with large intake cross-sections are indispensable. The inlet arrangements for delivering the material from the feeder into mill call for particular attention. One solution is provided by the big vertical inlet pipe, with a smooth lining to promote sliding of the feed material, passing through the center of the classifier. In that case, the feeder and bin must be mounted above the classifier. Considerable headroom inside the building is saved by introducing the raw coal sideways into the mill by means of screw feeder.

4.9 Control Behavior of ZGM Coal mills

ZGM mills can be controlled within the range from 25 percent to 100 percent of throughout capacity.

Central grinding plants are operated at a constant throughput rate, which can be adapted to inter-mediate storage capacity for the pulverized coal.

The mill throughout of direct-firing systems for boiler furnaces and rotary kilns is governed by the fuel feed rate requirements of the burners.

Control comprises three control loops:

-Raw coal feed
-Air flow rate through the mill
-Temperature in the classifier

The coal feed control loop is given its set point by the load controller of the boiler. The feed rate is varied by adjustment of the speed of the raw coal feeder.

In practice, when the coal feed rate to a direct-firing system is varied, the rates of air flow though the mill is varied proportionally between 70 percent and 100 percent. Tests have shown that continuous adjustment of the airflow rate over the mill loading control range considerably improves the control behavior of roller grinding mills. In addition, and anticipatory lead can be applied to the fuel control and air control action.

4.10 The aim is operational reliability

Besides the layout of the grinding plant, the machines with which the system is equipped are a major factor governing its operational reliability. The criteria applicable to the selection of all the components are:

-Economy of the plant as a whole
-Mechanical dependability in continuous operation
-Efficient handling and conveying of the materials, avoiding the formation to deposits of coal and coal dust.
-Easy maintenance and rapid remedying of faults
-Simple cleaning of the plant

4.11 Maintenance

Maintenance of ZGM roller grinding mills comprises:

-Regular inspection
-Changing the oil of the bearing in which the rollers are mounted, changing the oil in the gearbox
-Replacement of worn parts
-Removing reject material (if this operation is not mechanized).

Changing the grinding elements can be accomplished simply and quickly with special tools, via the hinged housing segment without disassembling the housing or ducts